Magnetic recording media are widely used as recording tape, video tape, computer tape, disc, etc. The recording density of magnetic recording media has become higher and higher, and the recording wavelength has become shorter and shorter every year. An analog system and digital system have been studied as the recording system for such magnetic recording media. In order to meet the high density requirement, a magnetic recording medium comprising a thin metal film as a magnetic layer has been proposed. A so-called coating-type magnetic recording medium comprising a magnetic layer that dispersed of a ferromagnetic powder in a binder coated on a support is advantageous in terms of practical reliability such as productivity and corrosion resistance. However, the coating type magnetic recording medium has a low packing density of magnetic powder as compared to a thin metal film and thus exhibits poor electro-magnetic characteristics. One widely used coating-type magnetic recording medium comprises a nonmagnetic support having a magnetic layer containing a ferromagnetic iron oxide powder, Co-modified ferromagnetic iron oxide powder, CrO.sub.2 powder, ferromagnetic alloy powder, etc. dispersed in a binder coated thereon.
To enhance the electro-magnetic characteristics of the coating-type magnetic recording medium, various approaches, such as improvement of magnetic properties of the ferromagnetic powder and smoothening of the magnetic layer surface, have been proposed. However, these approaches do not adequately enhance the recording density.
As noted above the recording density of the magnetic recording media becomes higher, while the recordable wavelength thereof becomes shorter. Therefore, if the thickness of the magnetic layer is great, the output is extremely decreased because of self-demagnetization during recording or reproduction.
In order to cope with these problems, the thickness of the magnetic layer has been reduced. However, if the thickness of the magnetic layer is about 2 .mu.m or less, the surface of the magnetic layer can be easily influenced by the nonmagnetic support, thereby deteriorating the electro-magnetic characteristics or worsening the output. The rough surface of the nonmagnetic support can be eliminated by providing a nonmagnetic thick undercoating layer on the support before coating the magnetic layer as described in JP-A-57-198536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, this solution is disadvantageous because of inferior head abrasion and durability. These inferior properties probably attributable to the use of a thermosetting resin as a binder in the lower nonmagnetic layer that results in a hardened lower layer, which in turn produces a magnetic recording medium that is less flexible. Contact of the magnetic layer with the head and other members without buffering abrases the magnetic layer. These problems can be possibly eliminated by using a nonsetting resin as a binder in the lower layer. However, this causes an additional problem; if a magnetic layer is coated as an upper layer after a lower layer coated is dried, the lower layer swells because of an organic solvent in the coating solution of the upper layer. This in turn causes turbulence in the coating solution of the upper layer, deteriorating the surface properties and hence the electro-magnetic characteristics of the magnetic layer.
The reduction of the thickness of the magnetic layer can be possibly accomplished by reducing the coated amount of the coating solution or adding a large amount of a solvent to the magnetic coating solution to lower the concentration thereof. If the former approach is used to reduce the coated amount of the coating solution, the coat layer begins to dry without sufficient leveling, causing coating defects, such as remaining of linear or marking patterns and hence considerably reducing yield. If the latter approach is used to lower the concentration of the magnetic coating solution, the resulting coating layer has many voids, causing various problems. For example, a sufficient packing density of magnetic powder cannot be obtained. Furthermore, the coating layer has an insufficient strength. The invention according to JP-A-62-154225 has a great disadvantage because the yield of the product is so poor.
As an approach for solving these problems, the applicants proposed a magnetic recording medium that has excellent productivity without coating defects and that exhibits improved reproduced output, electro-magnetic characteristics (e.g., C/N) and running durability by employing a simultaneous multi-layer coating process described in JP-A-63-191315 and JP-A-63-187418 to coat an upper magnetic layer comprising a ferromagnetic powder on a lower nonmagnetic layer while the nonmagnetic layer is wet.
However, the following problems cannot be solved even by applying such an approach.
In recent years, magnetic recording media have been required to attain a considerably high surface smoothness to reduce the spacing loss with the head in order to meet the requirements for high recording density and high output. To this end, the lower nonmagnetic layer, which is not externally visible, is also required further to exhibit dispersibility as high as possible and a high surface smoothness if the simultaneous multi-layer coating process is used. In the above mentioned simultaneous multi-layer coating technique, the surface smoothness of the magnetic layer can be possibly improved by using finely divided grains in the lower nonmagnetic layer to attain sufficient surface properties. However, such finely divided grains can easily agglomerate, deteriorating the surface properties of the lower nonmagnetic layer and hence the magnetic layer. This approach is also disadvantageous in that even if the thickness of the magnetic layer is further reduced to improve the electro-magnetic characteristics, the poor dispersibility of the powder in the lower nonmagnetic layer makes it difficult to control the interface of the magnetic layer with the lower nonmagnetic layer, distorting the interface and hence making it impossible to obtain a uniform magnetic layer. In other words, the thinner the magnetic layer is, the greater is the contribution of the dispersibility of the lower nonmagnetic layer to the surface properties of the magnetic layer obtained by the simultaneous multi-layer coating process. The conventional techniques cannot provide proper solutions.
Further, as the surface area of finely divided grains increases, they adsorb or absorb the lubricant contained in the magnetic layer or nonmagnetic layer, remarkably reducing the amount of the lubricant in the surface of the magnetic layer. This aggravates jitter and still life drop, as the friction coefficient of the magnetic recording medium increases.
Moreover, a known approach for improving the dispersibility of a nonmagnetic powder comprises the surface-treatment of the nonmagnetic powder with a known treating agent, such as polyol (e.g., pentaerythritol, trimethylolpropane), organic acid (e.g., aliphatic acid), alkanolamine (e.g., triethanolamine, trimethylolamine) and silicon compound (e.g., silicon resin, alkylchlorosilane) if the nonmagnetic powder is TiO.sub.2, silica or the like. These surface-treatments improve the dispersibility of the nonmagnetic powder but provide no improvements in the running durability of the magnetic recording medium. Furthermore, if a lubricant is used in a large amount to improve the running durability of the magnetic recording medium, it results in a reduced film strength.
Thus, the nonmagnetic powder to be used in the magnetic recording medium needs an improved dispersibility. However, no appropriate means have been found.